Earthquakes are the result of the Earth's response to stress building up in the crust. When accumulated stress exceeds a fault's frictional resistance to movement, ratoppid displacement occurs along the fault. This slippage releases seismic energy, which propagates radially from the earthquake source, resulting in the oscillating motion, or shaking, recorded by seismographs stationed around the world. Check out the Animated Earthquake Guide on the BBC webpage for graphics of earthquake mechanisms.

Seismic energy is released in the form of waves that are divided into two groups: body waves and surface waves. Body waves are further divided into Primary (P) and Secondary (S) waves based on wave form. P- waves travel fastest and are compressional-type waves, similar to sound waves. S-waves arrives more slowly and have a shear-wave form. The body waves are followed by the surface waves, which have the highest recorded amplitudes and cause the most noticeable ground shaking. Seismologists use seismometers to measure the amplitude of these waves to estimate the amount of energy released by the earthquake. The higher the amplitude, the larger the earthquake. Additionally, the distance between the earthquake and seismometer can be estimated by measuring the difference between the P and S wave arrival times and using estimates of the wave velocities. When such distances are calculated for multiple (three or more) seismometers, seismologists can determine the earthquake epicenter.

Seismogram recorded from a magnitude 6.5 earthquake in Columbia on January 19, 1995.
Image courtesy of Mark A. Horrell, Ph.D

Geologists work together with seismologists and engineers to study and understand earthquakes and the mechanisms that produce them. Geologists map the locations of faults, evaluate when and how often large earthquakes have occurred on the faults, and assess their probability of generating large earthquakes in the future. Also, geologists identify areas susceptible to geologic hazards associated with earthquakes, such as the presence of unconsolidated soil that can intensify shaking or be subject to liquefaction (described below).

Ground Motion - shaking of the ground as seismic waves propagate away from the earthquake epicenter. Ground motion is most intense closest to the epicenter and dissipates with increasing distance from the epicenter. Additionally, ground motion intensity can be a function of the subsurface soil and rock conditions. An unconsolidated material will shake more than solid rock. Ground motion is measured as a percent of Earth's gravitational acceleration (denoted as g). For example, ground motion that has a maximimum or "peak" acceleration equal to 25 percent of g will be denoted as 0.25g.

Map of estimated peak ground accelerations for the U.S. Image courtesy of USGS

Liquefaction - the loss of strength in water-saturated, unconsolidated soils caused by ground shaking. The shaking reduces grain-on-grain contact and subsequent frictional resistance, causing the unconsolidated materials to behave as a fluid rather than a solid. Liquefaction can undermine and severely damage buildings, roads and other structures.

Right: Photograph of liquefied soil along a railroad track. Photograph courtesy of USGS.

Tsunami - displacement of faults along the ocean floor result in long wavelength, low amplitude waves which move through the ocean and can develop into large, powerful waves as they make landfall. These are often, incorrectly, called tidal waves. For more information, see the Tsunami page.

Seiche - an oscillating wave that occurs in an enclosed or partially enclosed basin (lake, bay, gulf, and harbor), due to a disturbance such as the passing of a seismic wave through the basin.

Landslides - ground shaking can trigger landslides along marginally stable slopes and/or areas composed of loose, unconsolidated materials. Forces exerted on soil and rock during earthquakes can cause failure of slopes that remained stable during periods of no, or less severe, seismic activity. For more information, see the Landslides page.

Numerous earthquakes throughout the world have been recorded in human history. Some notable recent earthquake events include:

San Francisco Earthquake of April 18, 1906: No seismographs were available 1906, but scientists currently estimate the magnitude of the earthquake to be approximately 7.8 based on anecdotal evidence. This earthquake resulted in the deaths of 700 people and a cost of damage between $350 million to $1 billion in 1906 dollars. Most damage was the result of fires due to gas lanterns knocked over from shaking. Additionally, ruptured water lines greatly reduced the amount of water available to extinguish the fires. This earthquake fueled the study of seismology, resulting in an extensive investigation of the San Andreas Fault and associated geology (Rahn, 1996). Check out the USGS for additional details.

Alaskan Earthquake of March 27, 1964: This magnitude 8.4 earthquake occurred approximately 130 km east of Anchorage, and resulted in 130 deaths and $300 million to $750 million in damages. Extensive secondary hazards were associated with this earthquake, including a tsunami that caused damage to islands in the South Pacific. Additionally, faulting raised the land surface up to 12 meters and caused up to 2 meters of sea floor subsidence. Liquefaction of a poorly consolidated soil caused the overlying clay material to produce extensive landslides in the Turnagain Heights area of Anchorage (Rahn, 1996). Check out the USGS for additional details.

Loma Prieta Earthquake of October 18, 1989: Also known as the World Series Earthquake, this magnitude 6.9 event occurred along the Santa Cruz Mountain section of the San Andreas Fault previously noted as a seismic gap. The earthquake produced severe shaking in the San Francisco Bay area and liquefaction-related ground failure in the Marina District. The event resulted in 63 deaths, 8,000 homeless, and damage costing approximately $5.6 billion. Check out the USGS for additional details.

Pakistan Earthquake of October 8, 2005: Located in the Kashmir region of eastern Pakistan, this magnitude 7.6 earthquake resulted in the deaths of at least 86,000 people and left more 4 million homeless. The area is seismically active as the convergent boundary between the Indian and Eurasian tectonic plates (large segments of the earth's crust). In addition to the high energy ground shaking, this earthquake produced numerous landslides and rock falls, liquefaction, and seiches. Seiches generated from the event occurred as far east as Bangladesh (USGS).